In recent years, lithium secondary batteries have been widely used as driving power supplies for small electronic devices and the like. Such lithium secondary batteries are mainly constituted of a positive electrode comprised of a lithium compound oxide, a negative electrode comprised of a carbon material or a lithium metal, and a nonaqueous electrolytic solution. As the nonaqueous electrolytic solution, a carbonate such as ethylene carbonate (EC) and propylene carbonate (PC) is used.
A lithium secondary battery using, for example, LiCoO2, LiMn2O4 or LiNiO2 as a positive electrode material brings about a reduction of the battery performance, when part of the solvent of the nonaqueous electrolytic solution locally undergoes an oxidative decomposition during the charging, because the decomposition products inhibit the desired electrochemical reaction of the battery. Such a reduction is considered to be attributed to an electrochemical oxidation of the solvent at the interface between the positive electrode material and the nonaqueous electrolytic solution.
Also, a lithium secondary battery using, for example, a highly crystallized carbon material, such as natural graphite and artificial graphite, as a negative electrode material brings about a reduction of the battery performance, when the solvent of the nonaqueous electrolytic solution undergoes a reductive decomposition on the surface of the negative electrode during the charging. Even in the case of EC which is widely generally used as a solvent for the nonaqueous electrolytic solution, a part thereof undergoes a reductive decomposition during repeated charging and discharging.
As techniques for improving the battery characteristics of such lithium secondary batteries, there are known, for example, Patent Documents 1 to 9.
Patent Document 1 discloses a nonaqueous electrolytic solution for a secondary battery composed of an electrolyte and a non aqueous solvent containing a cyclic carbonate having a nonconjugated unsaturated bond, such as vinylethylene carbonate (VEC), in an amount of 0.1 to 20% by weight based on the entire nonaqueous solvent and suggests an improvement of cycle life as its feature. The battery containing VEC, however, has a problem that a gas due to decomposition of the electrolytic solution is generated at the negative electrode in a larger amount as compared with a battery without VEC, thereby to cause a reduction of the battery performance.
Patent Document 2 discloses a lithium secondary battery using a mixture of an ethylene carbonate derivative, such as VEC and monofluoroethylene carbonate, and triphenyl phosphate. With such an electrolytic solution system, however, satisfactory cycle characteristics are not obtainable. Further, sufficient initial capacity and cycle characteristics cannot be obtained when the charge final voltage of the battery is higher (4.3 V or higher) than the conventional one.
Patent Documents 3 to 6 disclose a nonaqueous electrolytic solution for a lithium secondary battery containing an alkyne derivative.
Patent Document 7 discloses a coin-shaped battery as a lithium secondary battery containing a pentafluorobenzene compound, such as pentafluoroanisole, having an electron donating group. The coin-shaped battery, however, shows retention of capacity after 200 cycles of only about 80% and, therefore, has insufficient cycle property.
Patent Document 8 discloses that pentafluoroanisole is usable as an oxidation reduction reagent as chemical means for protecting a nonaqueous electrolytic solution secondary battery from overcharge but does not mention the cycle property thereof. Patent Document 9 discloses a nonaqueous electrolytic solution for a lithium secondary battery containing a pentafluorophenyloxy compound.
In these nonaqueous electrolytic solutions, the cycle property, etc. are improved in a certain degree. However, further improvement of battery performance is needed.    [Patent Document 1] Japanese Patent Application Publication 2000-40526    [Patent Document 2] U.S. Patent Application Publication 2003/157413    [Patent Document 3] Japanese Patent Application Publication 2000-195545    [Patent Document 4] Japanese Patent Application Publication 2001-313072    [Patent Document 5] Japanese Patent Application Publication 2002-100399    [Patent Document 6] Japanese Patent Application Publication 2002-124297    [Patent Document 7] U.S. Patent Application Publication 2002/110735    [Patent Document 8] Japanese Patent Application Publication H07-302614    [Patent Document 9] Japanese Patent Application Publication 2003-272700